1. Field of the Invention
The present invention relates to a laser device which can be used as a laser light source for a laser printer, an optical disc drive, optical measurement, a laser display and the like.
2. Description of the Prior Art
Recently, a laser device including a wavelength converter (for second harmonic generation, sum frequency generation, difference frequency generation or the like) using a secondary nonlinear optical material have been studied to provide a light source for optical information processing (for example for an optical disk player and for a laser printer), for various optical measurement apparatuses and for a laser display.
Laser devices including a wavelength converter are mainly classified into following two kinds: (1) In a laser device of first kind, a solid laser medium such as Nd:YAG or Nd:YVO.sub.4 is excited by a laser diode, and a second harmonic wave (hereinafter referred to also as SH light) is generated by providing a wavelength converter (mainly a second harmonic generator) in an optical resonator of a solid laser device. Such a device uses a wavelength converter of inner resonator type. (2) In a laser device of second kind, a fundamental wave emitted from a laser diode is entered directly into a wavelength converter to take out a second harmonic wave. Such a device used a wavelength converter of external resonator type or that of waveguide type.
The laser device of first kind can convert wavelength relatively easily. Further, though an optical power of one to a few watts is needed for a light source of a laser display, such a large optical power output can be obtained relatively easily with a second harmonic generator of inner resonator type. However, a prior art Nd solid state laser medium has an oscillation wavelength of about 1.06 .mu.m and has a second harmonic of green light of about 0.53 .mu.m. Therefore, a second harmonic of a shorter wavelength than 0.53 .mu.m. Though Nd:YAG crystal can generate laser light at 0.946 .mu.n, a oscillation efficiency is lower about one order than for wavelength of 1.06 .mu.m. Further, the oscillation efficiency depends on crystal temperature.
Recently, it is reported that laser media such as Cr:LiCaAlF.sub.6 (hereinafter referred to as LiCAF) and Cr:LiSrAlF.sub.6 (hereinafter referred to as LiSAF) having chromium as active ions can oscillate at a wavelength range between 0.7 and 1.0 .mu.m due to excitation by a laser diode. However, if these laser crystals are used for a wavelength converter of inner resonator type, it is needed to select an oscillation frequency according to a birefringent filter and a grating, so that an optical resonator has a complicated structure. Further, because they are fluoride crystals, they are liable to react with air and hard to be grown. Because a fluorescent life of such oxides and fluorides is about 100 .mu.sec, a laser light can be modulated only at about 10 kHz. However, if it is used for a record light source for an optical disk player or the like, it is required to modulate a light at a few MHz. Therefore, an external optical modulator which used an electro-optical or acousto-optic effect is needed newly.
A wavelength converter of inner resonator type using a laser diode is also proposed (for example, Harold D. et al.: IEEE J. Quantum Electronics, QE6, 356-360(1970)). However, a laser beam of an ordinary laser diode is emitted through a narrow waveguide (a section of 0.1 .mu.m * a few .mu.m), and the outgoing light has a large diverging angle. Therefore, If a lens or an output mirror is arranged outside a laser diode, a few tens % of the light reflected by the output side mirror returns to the waveguide in the laser diode, so that the amplitude of a fundamental wave in the optical resonator cannot be increased. Therefore, a laser device cannot be realized with a wavelength converter of high efficiency.
In a laser device of second type, a light of second harmonic of 0.4 .mu.m band can be realized because a wavelength of a laser beam of a laser diode is converted directly. A second harmonic of wavelength of 0.3 .mu.m or shorter may be generated if a light of a laser diode of a II-VI compound semiconductor such as CdZnSe, ZnSe or ZnMgSSe is used for a fundamental wave. However, a wavelength converter of external resonator type used in a laser device of the second type needs a complex wavelength control technique in order to coincide a wavelength of a laser diode with that of an optical resonator. Further, if a crystal itself for wavelength conversion is used as an optical resonator (for example, W. Lenth et al., Proceedings of SPIE, 1219, 21-29 (1990) and Japanese Patent laid open Publication No. 335586/1992), a very precise curve surface has to be formed on ends of the crystal, so that the laser device has a very high cost. Further, in a wavelength converter of waveguide type used for a laser device of second type, it is difficult to introduce a high power laser light into a section of a few .mu.m * a few .mu.m, so that a light source for laser display cannot be produced. Still further, it is difficult to introduce a light of laser diode into a wavelength converter of waveguide type efficiently and stably. A mechanism for stabilizing a wavelength is also needed in order to stabilize a wavelength of a light of a laser diode.